The invention relates to a seal between two parts moved relative to one another through their sealing gap-forming approximation, with an outer plastic layer on one of the sealing gap-forming, facing surfaces.
Seals or packings of this type are e.g. known from DE-A-33 26 852, DE-A-36 21 178, EP-A-0 109 823 and EP-A-0 267 559. By the trueing of the parts on one another, the plastic layer thereof serves to allow an automatic formation of a very narrow sealing gap. This literature discloses that difficulties are encountered in preventing a detachment of the plastic layer due to centrifugal forces and that it is not possible to obtain a sufficiently thin layer to bridge the gaps necessary as a result of manufacturing tolerances and allow a running in on the entire, desired surface.
To obviate the problem of separation or detachment due to centrifugal forces, according to EP-A-0 267 559 the plastic layer applied in a mould is only located on the inwardly directed faces of a rotary machine, so that the centrifugal forces have a pressing action on the plastic layer.
The known seals of the aforementioned type also lead to the disadvantage that when used on machines with varying thermal stresses, e.g. for boosting internal combustion engines, the sought narrow sealing gaps cannot be obtained as a result of the thermal insulation action of the porous plastic layer. This insulating action means that the part carrying the plastic layer cannot follow without a time lag the thermal expansions of the adjacent, other part occurring on putting the machine into operation and after switching it off. Thus, in a rotary machine, the surrounding casing cools first and then shrinks on the still hot, enclosed rotor, so that the plastic layer thereof is excessively ground or worn away. On heating the machine the casing firstly expands, so that compared with the still cold rotor correspondingly wide gaps occur on the seal.
The problem of the invention is to find a seal of the aforementioned type, which can be easily manufactured, which permits narrow sealing gaps and which has an improved, reliable action.